On optical recording media such as, for example, CD and DVD, data is stored in the form of pits following tracks. Recordable media, such as DVD-RAM or DVD-RW, often use land-groove track structures, where data is written either in the grooves or both in the lands and the grooves. The tracks are centered relative to the center of the optical recording medium, either in the form of concentric circles or as a spiral. Favorably, the center of the tracks corresponds to the axis of rotation of the optical recording medium upon playback in an apparatus for reading from and/or writing to optical recording media. However, due to manufacturing tolerances of the optical recording medium and/or the drive of the apparatus, during playback or recording the center of the tracks does generally not coincide exactly with the axis of rotation. In other words, the optical recording medium exhibits eccentricity. The eccentricity affects the accuracy of track counting during a track jump, and, therefore, increases the time needed for accessing a specific track on the optical recording medium. During a track jump, the tracking loop of the servo controller, i.e. the control loop which ensures that the light beam used for reading and/or writing remains centered relative to the track, has to be opened. Once the track jump is finished, the tracking loop is closed again. As long as the tracking loop is opened, the eccentricity causes interfering activity on the tracking error signal, without any indication whether the light beam is moving inwards or outwards. This leads to an erroneous track counting.
For optical recording media exhibiting a difference in reflectivity between lands and grooves, a solution for this problem is found in using two signals for counting the number of tracks which are being crossed during a track jump. The first signal is the tracking error signal, the second signal is a mirror signal. Both signals are obtained as a combination of photodetector signals. By monitoring the phase relationship between these signals, it is possible to determine whether the light beam is moving inwards or outwards while the tracking loop is opened. This allows to compute the number of tracks which are being crossed during a track jump with sufficient accuracy, even under the influence of eccentricity.
However, recordable optical recording media such as, for example, DVD+RW do only exhibit a very small or even no difference in reflectivity between lands and grooves in the unrecorded areas. During a track jump the system has to rely solely on the tracking error signal for track counting. This signal, however, does not give any indication on the interfering influence of the eccentricity during the track jump.
To alleviate the above problem, JP 10-112039 discloses a method to improve the random access stability of an optical disk memory device. The device is provided with an eccentricity detecting circuit, which discriminates the direction of the eccentricity of the disk and the eccentric acceleration from the tracking error signal. Eccentric acceleration here denotes the acceleration of a track relative to its mean position caused by eccentricity. When a track jump is to be performed, a track jump enable signal generating circuit determines the track jump start timing from the obtained eccentricity condition, the track jump direction, the number of tracks crossed during the jump, and the relationship with the current rotation speed of the disk. The track jump is then initiated with an optimum timing against the disk eccentricity.
It is an object of the invention to improve the methods known from prior art.